Question: Umaima did 18 fewer sit-ups than Christopher at night. Christopher did 48 sit-ups. How many sit-ups did Umaima do?
Answer: Christopher did 48 sit-ups, and Umaima did 18 fewer than that, so find the difference. The difference is $48 - 18$ sit-ups. He did $48 - 18 = 30$ sit-ups.